Roof structures of industrial buildings typically comprise roof or rafter beams which extend parallel to one another across the building in one direction and purlins mounted on the rafters which extend parallel to one another in directions normal to the rafters. Flexible vinyl material in sheet form is applied in strips to adjacent purlins so as to span the space between adjacent purlins. The vinyl forms a vapor barrier between the roof and the space in the building below the roof. The vinyl sheets are prevented from sagging into the space between adjacent purlins during roof construction by wires that are stretched between the purlins and which become a permanent part of the roof. The vinyl sheets are in turn overlaid with heat insulation material, such as fiberglass sheets, which in turn are covered with sheets of corrugated metal.
Examples of roof structures of this type and methods and apparatus for installing such structures are disclosed in my U.S. Pat. Nos. 3,559,914; 3,969,863; 4,047,345; 4,075,807 and 4,147,003.
As just stated, upon completion roofs of the type just described retain the network of wires that were mounted to the purlins for the purpose of preventing the strips of flexible insulation from sagging during roof construction. Once the roof is completed, however, these wires serve no purpose since the vinyl sheets and fiberglass insulation are then prevented from sagging by the fasteners that extend through the metal roof sheets, the fiber glass and the vinyl sheets into the purlins. Thus, though the support wires are useful in preventing the sheet material from sagging briefly during roof construction, they become superfluous after the roof structure has been completed. Furthermore, since the support wires are present throughout the roof they are quite costly to purchase and to install.
Accordingly, it is seen that were an apparatus and method to be devised for applying flexible sheet material to purlins during the construction of a roof without the need for wires of other permanent support structure to support the sheet material, a distinct advance in the art would be achieved. It is to the provision of such therefore that the present invention is primarily directed.